Method of extracting heat from dry geothermal reservoirs

ABSTRACT

Hydraulic fracturing is used to interconnect two or more holes which penetrate a previously dry geothermal reservoir, and to produce within the reservoir a sufficiently large heat-transfer surface so that heat can be extracted from the reservoir at a usefully high rate by a fluid entering it through one hole and leaving it through another. Introduction of a fluid into the reservoir to remove heat from it and establishment of natural (unpumped) convective circulation through the reservoir to accomplish continuous heat removal are important and novel features of the method.

States Patent [1 1 Potter et al. A

[45] Jan. 22, 1974 METHOD or EXTRACTING HEAT FROM DRY GEOTHERMALRESERVOIRS Inventors: Robert M. Potter; Eugene S.

Robinson; Morton C. Smith, all of Los Alamos, N. Mex.

The United States of America as represented by the United States AtomicEnergy Commission, Washington, DC.

Filed: Mar. 27, 1972 Appl. No.: 238,435

Assignee:

US. Cl 165/1, 165/45, 166/247 llnt. Cl. F28d 21/00 Field of Search165/1, 45; 166/247 [56] References Cited UNITED STATES PATENTS 3,640,3362/1972 Dixon 165/1 Primary Examiner-Charles Sukalo Attorney, Agent, orFirm-John A. Horan 5 Claims, 2 Drawing Figures WATER i lFLow SEDIMENTSCRYSTALLINE BASEMENT ROCK *THERMAL REGION 300C VERTICALLY ORIENTED CRACKPRODUCED BY HYDRAULIC FRACTURING PATENTED 3.786858 SHEET 1 BF 2SEDIMENTS WATERT lFLow CRYSTALLINE BASEMENT ROCK *THERMAL REGION L\IERTICALLY ORIENTED CRACK PRODUCED BY HYDRAULIC FRACTURING u R G wmm N5 L LO CL A E G K L N D E 0 l O P SI H M" M TH E C70 m M R L m MmR L lIAOL E m 8 R CU C V I u A RSN TA 2 D C SE N mW H V. E 4 n8 IJWHA IBH FE54RL V AITE -AT OH)G FEM 2 5 UDIH mv wm CT I 9 D WT kRlS m N W E 4MP ODOPATENTEBJANZZIQH sum 2 or 2 HYDRAULICALLY FRACTURED THIN VERTICAL DISCMETHOD OF EXTRACTING HEAT FROM DRY GEOTHERMAL RESERVOIRS The inventiondisclosed herein was made in the course of, or under, a contract withthe U. S. Atomic Energy Commissionv It relates to a method of extractingenergy from a dry geothermal reservoir.

BACKGROUND OF THE INVENTION Many regions are known in which volcanic orintrusive activity has occurred recently enough so that the geothermalgradient is still as high as 150' to 190 C/km (435 to 550 F/mile). Insuch regions, temperatures high enough to produce commercially usefulsteam exist within 2 to 3 km (6,600'to 9,800 ft) of the earths surface.In a few of these places (including northern Italy, New Zealand,northwestern Mexico, and both northern and. southern California) afortunate combination of geological events has caused the hot rock to benaturallypermeable or sufficiently fragmented so that it is accessibleto circulating ground water, and to be overlain by impermeable rockstrata which have prevented its rapid cooling by the free escape ofsteam or hot water. Where the overlying strata are penetrated locally bynatural fissures or by drilled holes, natural steam is avilable for theeconomical generation of power or for other uses.

Where natural steam is not produced, the exploitation of thesegeothermal reservoirs has not so far been undertaken, in spite of thefact that many of them are closer to the earth's surface than are thelower levels of a deep mine. In part this is because of the difficultyof drillingor tunneling into the hot, hard, crystalline rocks thatcompose most geothermal reservoirs. Principally, however, it is becausethe thermal conductivities of rocks are typically very low. Theirspecific heats are high, so thata relatively large amount of heat isavailable from a unit volume of the hot rock. This heat, however, can beextracted from the rock only through some free surface, such as the wallof a borehole. Since heat is conducted to that surface quite slowly, avery large surface is required if thermal energy is to be removed fromthe rock at a usefully high rate. It has generally been assumed that thecreation of the required amount of heat-transfer surface within a dense,crystalline rock is not practical by existing methods. In fact, thecommon oil-field technique of hydraulic fracturing appears to representa simple and practical method of developing the necessary new surface.

Hydraulic fracturing is a technique commonly used in the petroleum andnatural gas industries to create a system of cracks in the rock adjacentto a borehole. These cracks facilitate the flow of crude petroleum ornatural gas from the surrounding-formations into the well. Hydraulicfracturing is normally done by inserting temporary seals in the wellabove and below the zone to be fractured, perforating the casingsomewhere between these seals, and using a high-pressure pump to producehydrostatic pressure in this zone of the order of a few hundreds to afew thousands of psi above the horizontal component of the overburdenpressure. A crack system is created which may extend for many feet fromthe hole, the resulting increase in volume being accommodated locally bynatural porosity and by elastic compression of the uncracked rock.Carefully sized sand is usually injected with the fracturing fluid toprop the cracks open with a strong but permeable supporting material, sothat they willnot spring shut when the fracturing pressure is released.This technique of creating an extended crack system in deeply buriedrock has been used extensively in a wide variety of sedimentaryformations whose strength properties approach those of commoncrystalline rocks. For example, Halliburton, I971, cites hydraulicfracturing at 12,000 to 15,000 ft depth in the Ellenburger formation ofWest Texas, which is a strong, massive limestone having properties verysimilar to those of a granite. Because rocks are relatively weak intension and because the horizontal component of lithostatic pressure isgenerally much less than the vertical component, the fluid pressurerequired to produce fracturing is much less than might initally besupposed.

Another method of extracting geothermal energy is suggested in AProposal for a Nuclear Power Pro- 7 gram, by George C. Kennedy, USAECThird Plowshare Symposium, University of California at Davis (1964).This report discloses a nuclear device which would be detonated at thebottom of a hole creating a large, rubble-filled chimney of rock and aregion surrounding said chimney of fractured rock. In this report thewater is allowed to boil in the reservoir resulting in a marked decreasein fluid viscosity and therefore it is limited to the heat content ofthe initial rubblefilled cavity. Also a pressurized water cycle wasconsidered. This approach was abandoned because of potentially largeamounts of radioactive fission products that would be brought to thesurface by the circulating hot water and subsequently precipitated outon the tube wall surfacesof the power plant boiler.

SUMMARY OF THE INVENTION This invention states a means of extractingvery large amounts of thermal energy from the many regions of the earthssurface known to contain abnormally hot-- but essentially dryrock atdepths presently attainable using conventional drilling methods (todepths of the order of 20,000 ft or so). Dry is defined in thisapplication as not containing sufficient amounts of naturally occurringsteam or hot water to make these regions economically attractive asconventional (wet) geothermal energy sources.

After drilling into sufficiently hot rock, varying from about to 500 C,depending on both the economics of and proposed use for the heat, andthe costs of drilling; a very large heat transfer surface is created byhydraulically fracturing the surrounding rock at or near the bottom ofthe hole. The fracture system thus formed will normally be in the formof a very large but thin vertical circular disc (actually an oblatespheroid), with a radius of the order of thousands of feet. However, thefracture system may also be in the form of multiple vertical circularcracks radiating out from the well bore.

. The upper portion of the fracture system will then be connected to thesurface with a shallower drilled hole (or by a concentric, insulated,counter-current flow passage in the initial drilled hole). A circulatingwater loop will then be established: down the deeper hole, through thefracture system, up the shallower hole to the surface, and through theprimary heat exchanger of a suitable power plant.

GENERAL DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of oneembodiment of this invention.

FIG. 2 is another schematic view of this invention showing a secondembodiment employing concentric pipes'to circulate a fluid through ageothermal reservoir.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The geothermal system of thisinvention is shown schematically in FIG. 1. The upper parts of bothholes, through the sedimentary and/or volcanic sections are drilled 17%inches in diameter and lined with 13% inch steel casing, cemented inplace. The first (deeper) hole is extended at a 12 inch diameter to adepth of about 14,800 ft or until a rock temperature of about 300 C isencountered. This section is cased with 9% inch casing 1. The hole iscontinued for an additional 200 ft or so withan 8% inch bit, and thislast section of hole is left uncased. At a point about 1,200 ft abovethe bottom of the hole, the casing is jet perforated. A string of 7-inchhigh-pressure tubing would be run down the hole, which would bepacked-off above the perforated zone. The crystalline rock is fracturedhydraulically, and the resulting crack 2 extends out to a radius ofabout 1,500 to 4,000 ft. The second drill hole 3, at a location chosento encounter the upper part of the initial hydraulically fractured cracksystem, is drilled through the crystalline rock at a 12-inch diameteruntil the desired intersection occurs. If circulation to the first holehas not then been established, directional drilling would be used fromthis point to probe for the crack system 2. The cold water pipe inflowsystem 1 is connected through the vertical hydraulically fracturedcrystalline basement rock 5 to the return hot water pipe system 3 whichin turn is connected to a heat exchanger, turbine means, and aconventional power plant 4 at the surface.

Alternatively, or as a supplement to directional drilling, hydraulicfracturing could be repeated from the bottom of the second hole. A stillfurther solution to this problem is shown in FIG. 2. The cold water ispumped down an inner pipe insulated by any well known means from thereturn heated water flowing up the outer pipe. Thus by placing a stringof pipe within a larger pipe communication with the reservoir would beassured. In particular the inner pipe 1 having a viscous oil filling theannular gap 6 would be in communication with a hydraulically fracturedthin vertical disc 2 which in turn is in fluid communication with outerreturn pipe 3, a suitable heat exchanger and power plant 4.

When the underground circulation system is completed, a heat-exchangeris installed at the surface, capable of extracting 150 MW of thermalenergy from pressurized water entering it at 280 C and leaving it at 65C. Removal of heat from the geothermal reservoir at this rate requires awater flow of only about 315 lb/sec, which is significantly less thanthe natural convective flow capability of the piping.

The following Table depicts a typical geothermal reservoir located inthe Western United States:

RESERVOIR CHARACTERISTICS Rock Type: Granite or other crystallinebasement rock K 0.006 cal/cm-sec- C p 2.7 g/cm Cp 0.19 cal/g- CI-Iydraulically Fractured Crack:

Radius 1.6 km (5,250 ft) Volume -82,000 m (21.6 X 10 gal) Surface Area16 km (0.17 X 10 ft Depth to Center of Reservoir: 5 km (16,400 ft) RockTemperature at Center of Reservoir: 300 C (572 F) Geothermal Gradient(Assumed):

For cryatalline basement rock: 45 C/km For overlying sedimentary rock:C/km 2 km at K 0.0036 cal/cm-sec- C) CASED AND DRILLED HOLE SIZES ANDDEPTHS (Conventional oil field casing and drill bit sizes are assumed)Injection (deeper) Hole:

Depth 5 km (16,400 ft) Upper Half: 17 /2 inches drilled hole, 13% inchescasing Lower Half: 12% inches drilled hole, 9% inches casing Withdrawal(shallower) Hole:

Depth 3.5 km (11,500 ft) Upper Two-thirds: 17 /2 inches drilled hole,13%

inches casing Lower One-third: 12% inches open hole RESERVOIR THERMAL/FLOW POTENTIAL Reservoir Lifetime: 10 years (Excluding any contributiondue to thermal stress cracking) Average Pressurized Water Flow Rate: 265kg/sec* (This is 6 X 10 gal/day at the earth inlet) Earth InletConditions: T= 65 C (149 F) P 70 kg/cm (1,000 psia) Earth OutletConditions (Average):

T 280 C (536 F) P kg/Ctn (1,140 psia) Average Thermal Power: 250 MW[Potential Electrical Power Generation 50 MW] (at a net efficiency of 20percent) *Natural convection only (no pumping) conditions averaged over10 year lifetime.

The original heat transfer surface area of the reservoir (thehydraulically fractured disc) is augmented by additional heat transfersurface area resulting from thermal stress cracking as the surface ofthe original reservoir cools. Removal of heat from a body of rockresults in a volume contraction, AV, given by -AV'=' 3H H/cp, where H isthe linear coefficient of thermal expansion in C, c is the heat capacityof the rock in cal/g- C, and p is the rock density in g/cm This thermalcontraction will result in fracturing of rock adjacent to the primarycrack. Calculations have indeed shown that the rate of reservoir heatremoval (or reservoir power level) will pass through a minimum and thenincrease beyond the initial reservoir heat removal rate due tosubsequent thermal stress cracking of the reservoir rock. This reservoirextension phenomenon is due to a great extent to the viscosity variationof water by over a factor of five, between the reservoir inlettemperature 65 C) and the hotter portions of the reservoir 300 C), sothe pressurized water will tend to preferentially flow toward the hotterportions of the reservoir.

For a reservoir depth of 15,000 ft, there is a pressure difference ofabout 1,500 psi between the descending cold water column and theascending hot water column. This pressure difference arises from the 21percent density difference between the cold and hot water columns. Thus,this AP is available to overcome fluid friction losses in the piping andheat exchanger, eliminating the need for a circulating pump in thepressurized water loop.

The essential novel features of the method disclosed herein are directedto the fact that thermal stress cracking of the reservoir rock as heatis removed by the convective flow of pressurized water would produce acontinually enlarging crack system so as to significantly exnot seektemperatures in excess of 500 C because of 25 prohibitive drillingcosts.

What we claim is:

1. A method of extracting energy from a dry igneous rock geothermalreservoir comprising:

a. drilling a hole to such a depth as is required to encounter hotigneous rock in the range of to 500 C;

b. hydraulically fracturing from this hole to produce a large cracksystem in the rock;

c. pumping cold fluid down the hole to establish underground circulationthrough the crack system;

d. extracting thermal energy from the pressurized hot fluid rising in ashallower flow passage and then to a heat-exchanger at the surface; and

e. re-introducing the cooled fluid from the heatexchanger into the cracksystem.

2. The method of claim 1 in which the said cold fluid causes a secondcrack system to develop which is in communication with the initial cracksystem.

3. The method of claim 1 in which the said hole contains both the coldand hot fluid flow passages.

4. The method of claim 1 in which the temperature of said hot rock isabout 300 C.

5. The method of claim 1 in which the said fluid is water.

1. A method of extracting energy from a dry igneous rock geothermalreservoir comprising: a. drilling a hole to such a depth as is requiredto encounter hot igneous rock in the range of 150* to 500* C; b.hydraulically fracturing from this hole to produce a large crack systemin the rock; c. pumping cold fluid down the hole to establishunderground circulation through the crack system; d. extracting thermalenergy from the pressurized hot fluid rising in a shallower flow passageand then to a heat-exchanger at the surface; and e. re-introducing thecooled fluid from the heat-exchanger into the crack system.
 2. Themethod of claim 1 in which the said cold fluid causes a second cracksystem to develop which is in communication with the initial cracksystem.
 3. The method of claim 1 in which the said hole contains boththe cold and hot fluid flow passages.
 4. The method of claim 1 in whichthe temperature of said hot rock is about 300* C.
 5. The method of claim1 in which the said fluid is water.